Protractor for drafting machine



May 5, 1964 e. R. SNURR 3,131,478

PROTRACTOR FOR DRAFTING MACHINE Filed July 25, 1959 4 Sheets-Sheet 2 G. R, SNURR PROTRACTOR FOR DRAFTING MACHINE May 5, 1964 Filed July 23, 1959 5 4 Sheets-Sheet 5 a 4 ATTO UH Q L.

y 1954 G. R. SNURR 3,131,478

PROTRACTOR FOR DRAFTING MACHINE Filed July 23, 1959 4 Sheets-Sheet 4 a0 40 4g 34 J9 V 3 as 3 26 4/ I 11 E 5 9 /J j TI E 7 I. 1 1'4 //11 United States Patent 3,131,478 PRO'IRACTOR FOR DRAFTING MACHINE Gordon R. Snurr, Waynesboro, Pa, assignor to Glideline Corporation, Wayneshoro, Pa., a corporation of Pennsylvania Filed July 23, 1959, Ser. No. 828,984 8 Claims. (Cl. 33-79) 'My invention relates broadly to drafting machines and more particularly to a drafting machine protractor assembly which provides automatic indexing every 15 through a full 360 by push-button action as well as provisions for setting and locking the protractor at intermediate azimuth angles.

One of the objects of my invention is to provide a construction of drafting machine protractor which gives push-button operated automatic, positive indexing every 15 through 360.

Another object of my invention is to provide a construction of drafting machine protractor in which automatic indexing is easily disengaged for intermediate angle settings.

Another object of my invention is to provide a simple construction of azimuth ring lock, adjustable to compensate for wear, for a drafting machine protractor assembly.

Still another object of my invention is to provide a' construction of drafting machine protractor in which the index pins are raised and lowered into the index ring by a sliding incline plane arrangement.

A further object of my invention is to provide a construction of drafting machine protractor in which the protractor assembly is selectively locked to the drafting machine vertical rail by a simple spring biased pivoted lock ring disposed about the rail and cocked at acute angles with respect to the rail into engagement with the vertical rail.

Other and further objects of my invent-ion reside in the construction of the automatic indexing mechanism and the protractor vertical rail locking mechanism as set forth more fully in the specification hereinafter following by reference to the accompanying drawings, in which:

FIG. 1 is a top plan view of the drafting machine protractor of my invention;

FIG. 2 is a fragmentary bottom plan view of the protractor assembly;

FIG. 3 is a fragmentary top plan view of the protractor assembly of FIG. 1 with the center hand knob and lifting mechanism removed, the left side of the figure being in cut-away section to show the locking mechanism engaged with the vertical rail;

FIG. 4 is a cross-sectional view taken substantially along line 4-4 of FIG. 1 and particularly showing the incline plane trigger actuator for the protractor locking mechanism;

FIG. 5 is a cross-sectional view taken substantially along line 5-5 of 'FIG. 1;

FIG. 6 is a cross-sectional view of the hand knob and trigger actuator mechanism of the protractor assembly, particularly showing, in elevation, the trigger and associated incline plane in the locked position;

FIG. 7 is a view similar to FIG. 6 but showing the trigger actuator in the locked position;

FIG. 8 is a transverse sectional view taken substantially along line 8-8 of FIG. 7;

FIG. 9 is a cross-sectional view taken substantially along line 9-9 of FIG. 7 and particularly showing the protractor locking mechanism in the unlocked position;

FIG. -'10 is a top plan view of the locking mechanism index detent plate;

FIG. 11 is a vertical sectional view taken on line 11-11 of FIG. '10;

FIG. 12 is a side elevational view of the protractor central swivel stud member;

FIG. 13 is a top plan view of the protractor locking mechanism trigger actuator member;

FIG. 14 is a vertical sectional view taken on line 14-14 of FIG. 13;

FIG. '15 is a cross-sectional view taken substantially on line 15-15 of FIG. 3 and particularly showing the vertical rail lock mechanism in the locked position;

FIG. -16 is a transverse sectional view taken substantially along line 16-16 of FIG. 3;

FIG. 17 is a transverse sectional view through the protractor assembly vertical r-ail locking mechanism similar to the fragmentary cut-away portion of FIG. 3, but showing the mechanism in the unlocked position;

FIG. 18 is a view taken substantially along line 18-18 of FIG. 17;

FIG. 19 is a cross-sectional view taken substantially along line 19-19 of FIG. 17;

FIG. 20 is a transverse sectional view taken substantially along line 20-20 of FIG. 17;

FIG. Q1 is a top plan view of the lock ring member of the vertical rail locking mechanism;

FIG. 22 is a side elevational view of the lock ring member of FIG. 21;

FIG. 23 is a cross-sectional view taken on line 23-23 of FIG. 21;

FIG. 24 is an elevational view of a fragmentary end portion of a drafting machine scale, particularly showing the scale mounting plate;

FIG. 25 is a longitudinal sectional view taken substantially on line 25-25 of FIG. 24; and

FIG. 26 is a cross-sectional view taken substantially on line 26-26 of FIG. 24.

My invention is directed to a construction of protractor assembly for a dmafting machine which provides automatic indexing every 15 through a full 360 for faster protractor manipulation precision accuracy. In the protractor of my invention the base plate carrying detachable scales, the protractor azimuth ring and the protractor hand knob all rotate as a with respect to the protnactor housing about a central spindle connected to the housing through a bearing assembly. A push-button type trigger protrudes from one side of the hand knob and through an incline plane mechanism interiorly of the hand knob, as the trigger is depressed it raises a lifting plate to which index pins are rigidly connected. The index pins slide up and down in hearings carried by an index detent plate immediately below the hand knob and movable with the hand knob. An index ring carrying tapered slots is located immediately below the detent plate integral with the protractor housing, the index ring slots being arranged to receive the index pins and lock rotation of the hand knob at 15 increments when the trigger is released, thus giving automatic accurate and quick 15 indexing without having to pause to accurately set the azimuth dial with respect to the matching vennier which are viewable through a cut-out provided in the housing surface. Thus, whenever rthe trigger is depressed the index pins are lifted from the index ring slots and the protractor is free to rotate to a new setting.

To disengage the automatic 15 indexing as the trigger is depressed it is rotated slightly in a clockwise direction thus locking the index pins in the disengaged position with respect to the index ring slots. Now the protractor can be set to any intermediate angle and can be locked at that setting by an azimuth ring clamp which engages the bottom of the azimuth ring and applies pressure thereto so as to lock the azimuth ring to the protractor housing when areas? 3 the thumb clamp is given a slight clockwise rotation. To release the azimuth ring for a new protractor set-ting the clamp is given a slight counterclockwise rotation. While automatic indexing is being utilized the azimuth rin g clamp is in the unlocked position. 7

The trigger is spring-loadcd so that when it is released from the locked state it automatically springs outwardly and returns the index pins to the index ring slots. This is accomplished by a spring-loaded plunger member directing an outward force to the trigger and an index booster spring disposed interiorly of the hand knob and directing a downward force on the lifting plate and the incline plane mechanism to force the trigger to slide down the incline plane and push the index pins into their associated slots.

The entire protractor assembly glides up and down the draiiting machine vertical rail. This is accomplished by the vertical rail passing through a ball bearing assembly lined bored protrusion integral with the protractor housing so that the only connections between the housing and the rail are rolling ball bearing contacts. At one end of the bored protrusion I provide a protr-aotor housing vertical rail locking mechanism to securely lock the housing at any desired point along the vertical rail. This is accomplished by cocking a spring-loaded lock ring, which encircles the rail, at an acute angle to the rail so as to grip the rail and prevent any movement between the housing carrying the lock ring through a pivotal connection and the vertical rail.

Referring to the drawing in greater detail, the housing of the protrac-tor assembly of my invention is designated at 1 carrying the bored protrusion In for engagement of the drafting machine vertical rail 2. The protractor ass'em-bly is arranged to travel up and down the vertical 2 of the draiiting machine. To provide smooth gliding movement of the protractor assembly as it travels along the vertical rail the interior bored portion of the protrusion 1a carries ball bushing units secured therein and disposed between the vertical rail 2 land the housing protrusion 1a. These ball bushing units carry ball bearings which provide rolling contact between the protractor assembly and the vertical roll, the ball bearings being arranged to insure a smooth slide fit Without shake or play. These ball bushing units preferably contain precision nylon ball bearings, thus eliminating the necessity of lubrication, insuring longer life and providing a quieter operating drafting machine.

The top end of bored protrusion 1a is substantially closed around the circumference of the vertical rail by spring plate 3 secured to the interior of the bore. The clearance provided between spring plate 3 and vertical rail 2 is very slight so that the spring plate can also act as a rail wiper for removing large particles of chaser dust, etc, from the rail and thus prevent its entrance into the ball bushing units and the vertical locking mechamsm.

Theve'rtic'al locking mechanism is located adjacent the spring plate, inte'riorly of the bored protrusion 1a, and consists of lock ring 4, lock ring guides 5 and 6, vertical lock springs 7 and lock lever 8. Lock ring 4 provides slots 9 and 10, o'ficenter With respect to each other, through the side walls thereof, as shown in FIGS. -17, l921 and 23, [for slid able engagement with lock ring guides S and 6 which are secured in the Walls of protrusion 1:: and protrude outwardly into the interior bore thereof into slots 9 and 10, respectively, of the lock ring. The top surface of the lock ring, adjacent the side con taining the slots, provides mounting holes 11 for springs 7. The bottom surface of spring plate 3 provides similar mounting holes oppositely situated from those on lock ring 4 and springs 7 are mounted in a compressed state be tween the spring plate and the lock ring in the mounting holes provided thereon.

Lock lever 8 is threaded into the side of the lock ring opposite the mounting holes :1 1 and protrude through the irregular shaped opening 12 in the side wall of protrusion In as more clearly shown in :FIG. 18. Irregular shaped opening 12 provides a high abutting edge 12a for lever 8 for the locked position of the locking mechanism as shown FIGS. 1, 3, 15 and 16, and a low abutting edge 12b for lever 8 for the unlocked position of the vertical rail locking mechanism as shownin FIGS. 17-20. In the locked position lever 8 abuts the edge 12a of opening 12, guides 5 and 6 are moved to a position in the ends of slots 9 and 10 as shown in FIG. 15, and springs 7 exert a downward force on lock ring '4 and pivot it about guides 5 and 6 to a cooked position against rail 2 as shown in FIGS. 3 and 1 6. The springs 7 thus hold the lock ring rigidly against the vertical rail in such a. manner that the protrao tor housing which carries the lock ring is rigidly locked from movement with respect to the rail 2. The interior edges of the lock ring are left sharp to enable them to grip rail 2. and thus hold the protractor assembly in an immovable position with respect to the rail, while the exterior edges of lock ring 4 are rounded to enable it to be cocked sideways in the bore of protrusion do without catching on the interior side walls thereof.

To unlock the protractor housing from its locked position on the vertical nail, the lever 8 is pushed downward and rotated slightly clockwise to a. position as shown in FIGS. 174.0. In this position the lock ring 4 has been pivoted about guides 5 and 6 into a horizontal position, thus further compressing springs 7. As the lock ring is rotated clockwise by lever 8, lever 8 comes into contact with the edge 12b of opening 12 and is held in abutment with this edge by the downwardly directed force of springs 7. The clockwise movement of lock ring 4 positions the guides 5 and 6 to approximately the center position of slots 9' and :10 as shown in FIG. 19, and moves the mounting holes for springs 7 in spring plate 3 and lock ring 4 oil-center with respect to each other thus putting a curve in the springs as shown in FIG. 20. In the unlocked position' the interior edges of lock ring 4 are moved out of Contact with the vertical rail and the protr'actor assembly is free to travel up and down the vertical rail.

P-rotractor base plate '13 carrying fork extensions 14 is secured to central swivel stud '15 by blots l6. Azimuth ring 17 is adjustably secured to base plate 13 by bolts 18 in elongated counterbored depress-ions 19. Central swivel stud 13 has a tiered design, the first tier of which provides a mounting shoulder for the base platecarrying the azimuth ring. The second tier of swivel stud 15 provides a shoulder for engaging the center rotatable ring of the ball bearing assembly 20, the outer casing of which is securely mounted to protractor housing 1. The fourth tier of swivel stud 15 is provided with a fine knurl about its periphery for securely engaging index detent plate 21 slipped on the end of said stud.

Index detent plate 2:1 provides holes 22 along the center line thereof, :from each other, for the mounting therein of guide bushings .23 for the slidable engagement with tapered index pins 24. Index pins 24 are secured on the top ends thereof to lifting plate Q5 by means of snap rings 26 around each pin on either side of the plate as shown in FIGS. 5, 8 and 9. Beneath index detent plate 21 secured ina recess in protractor housing 1, adjacent the top end of bearing 20, I provide index ring 27 having tapered slots 28 in the upper surface thereof for receiving the matching tapered ends of index pins 24 to assure positive indexing, finer angular accuracy, and longer machine life, since the tapered design compensates [for wear. The index ring 7, index pins 24 and guide bushings 23 are preferably made of hardened and ground tool steel. The tapered slots 28 on the index ring upper surtface are twenty four in number and are arranged at 15 on centers to give accurate 15 indexing through a full 360.

Hand knob 29 carrying center post 30 is rigidly mounted to the top surfiace of detent plate 2 1 by means Of Screw 31 threaded into the central swivel stud 15. The

mounting screw 31 is located in a recess in the hand knob center post and this recess is covered by protractor seal 32 to give the drafting machine band knob a finished appearance. The pro-tractor seal may also conveniently be used as a manufacturers nameplate for the drafting machine. From the assembly then it can be seen that hand knob 29, detent plate 21, central swivel stud 15, and base plate 13 carrying fork and azimuth ring 17 all move as a single unit, in bearing 2-9, with respect to the protractor housing 1 as the hand knob is rotated. To facilitate easy manipulation there is a slight clearance between the top surface of azimuth ring 17, on which the numeral azimuth dial is cut, and the protractor housing. There is also a slight clearance between the base plate 13 and the protractor housing. A substantial-1y dust-free rotary joint is formed between index detent plate 21 and the protractor housing by providing an overlapping edge shown at 3-3 on detent plate 2 1 which cooperates with a similar adjacent overlapping edge on the protractor housing.

T o raise and lower the index pins 24- out of and into the slots 23 of index ring 27 i provide a push-button action arrangement controllable by the left thumb which results in automatic, positive indexing every 15. A trigger member 34 protrudes from hand knob 29 through a slot 35 provided in the side wall thereof such that the trigger is easily engageable by the left thumb when the hand knob is grasped by the left hand. The trigger member contains an elongated slot as with a substantially horseshoe-shaped incline plane portion shown at 37 on the lower surface of one end thereof. Hand knob center post 39 protrudes through trigger slot 36 and also through the center of biting plate 2.5 and acts as a guide for both the trigger and the lifting plate. The top surface of the trigger is in sliding contact with the bottom surface of lifting plate 2 5 While the incline plane portion 37 of the trigger member is in sliding contact with inclined lifting cam 38 disposed in recess 39 in the top surface of index detent plate 21. Lifting cam 38 is preferably constructed of nylon to eliminate the necessity of cam lubrication and also to increase the operating life of the lifting mechanism. Index booster spring 46* is disposed about center post 30 and at all times is in compressed cont-act with lifting plate 25. Trigger member 34 is normally biased to the trigger-out position, or index locked position, by the plunger assembly shown at 4-1. As shown in FIG. 6 the plunger assembly consists of a plunger sleeve 42, embedded in detent plate 21, containing plunger spring 43 and depressible plunger head 44-. To raise the pins 24- from the slots 28 the trigger is pushed in resulting in the raising of lifting plate 25 and the further compression of spring 46 and the incline plane portion 37 of trigger 34 slides up the inclined lifting cam 38. Since the index pins are secured to the lifting plate 25, as the lifting plate is thus raised the pins 24- :are withdrawn from the slots 28. Also as the trigger is depressed the plunger head 4-4 is depressed, thus compressing spring 43. With the index pins raised the hand knob 29 is then turned either clockwise or counterclockwise, as desired, and then the trigger member released to drop the [index pins into the desired 15 interval slots, thus automatic, positive indexing on the 15 intervals. As the trigger is released the combined force of compressed spring 40 pushing down on the inclined surfaces and the compressed spring 43 pushing plunger head 44 out against trigger 34 return the trigger to the biased outward position, thus dropping the pins into the slots 28.

The protractor assembly is shown in the index locked position in FIGS. 1, 4-6 and in the index unlocked position in FIGS. 7-9. As shown in FIG. 8, if automatic 15 indexing is not wanted in the particular operation and intermediate angle settings are desired, the trigger member 34 is depressed, as explained previously, and the trigger is given a slight clockwise rotation as shown at 45 to engage the side wall of hand knob 29 in the slot 46 of the trigger member, thus locking the trigger member in the depressed position and the index pins in the raised or disengaged position. In this unlocked state the protractor can be set to any desired angle and can be locked at that intermediate setting by azimuth clamp 47. As shown in FIG. 5 the azimuth clamp consists of azimuth lock stud 48 secured by clamp 49 and its associated bearing washer, clamp nut 49 being adjustable by azimuth thumb clamp 50. The head of azimuth lock stud 48 engages a recessed groove on the under surface of azimuth ring 17 and protrusion 51 on either side of the lock stud head abut the edge of the azimuth ring adjacent the recessed groove to prevent the stud 48 from rotating when the clamp nut 49 is tightened on the stud by manipulation of azimuth thumb clamp 50. Thus when the thumb clamp tightens the nut 49 on stud 48, the head of stud 48 applies pressure so as to tightly sandwich the azimuth ring 17 between the stud head and the under side of protractor housing 1 resulting in the azimuth ring and thus the hand knob being locked so that they are unable to rotate. Azimuth thumb clamp 50 has a sliding fit with respect to clamp nut 49 so that as the under side of the azimuth ring and the stud head are reduced in thickness due to wear, all that is necessary to adjust the clamp to compensate for the wear is to slip thumb clamp 50 off of clamp nut 49, tighten clamp nut 49 to the desired new position and then replace the thumb clamp 50 thereon. Thumb clamp 50 is rotated counterclockwise to release the clamping pressure on the azimuth ring and the hand knob can then be rotated to set the protractor at a new angle. The azimuth clamp is set in the unlocked position when automatic indexing is being utilized.

Referring to FIGS. 1, 3 and 5, the protractor housing 1 provides a cut-out portion indicated at 52 through which the protractor scale or azimuth dial of azimuth ring 17 is visible, thus enabling the azimuth ring to be aligned with the double Vernier scale carried by a portion of the cut-out and indicated at 53. The only portion of the azimuth ring which is visible is that portion which is underneath the cut-out portion; thus the protractor housing protects the precision-cut protractor scale from dust and external bumps. By use of the double Vernier scale intermediate angle settings can be set to give readings to five minutes.

The bottom surfaces of fork extensions 14 of base plate 13 provide scale mounting tapered grooves 54 with upwardly tapered sides for receiving the adjustable mounting plates 55 secured by means of screws 56 and 57 to the upper surfaces of drafting machine scales 58.

Eyelet 59 is provided on the top edge of the protractor housing adjacent the protrusion 1a for attachment of a counterbalance cable from the horizontal rail head of the drafting machine.

I have constructed the protractor assembly of my invention and have used it on a drafting machine and have found it to be very accurate, rugged, useful, efficient, and reliable, very easily manipulated when using either the automatic indexing or the manual intermediate angle azimuth ring lock, and I have found all the locking mechanisms reliable and unyieldable under normal operating conditions.

While I have described the protractor of my invention in certain preferred embodiments I realize that modifications can be made and I desire that it be understood that no limitations upon my invention are intended other than may be imposed by the scope of the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States is as follows:

1. A protractor for a drafting machine comprising a protractor housing having a cut-out portion, a hand knob, a base plate journaled with respect to said protractor housing and rotatable by said hand knob, a protractor azimuth ring secured to said base plate between said base plate and said protractor housing, an azimuth dial carried by said azimuth ring, said azimuth ring being viewable through the cut-out provided in said protractor hous ing, said azimuth ring having a groove onthe bottom face thereof adjacent the inner edge thereof, and pressure applying means extending through said protractor housing inwardly of said azimuth ring and selectively engaging said azimuth ring groove for applying upward pressure thereto to frictionally clamp the face of said azimuth ring to said protractor housing to selectively lock said protractor at an intermediate angle setting.

2. A protractor for a drafting machine as set forth in claim 1, in'which' said pressure applying means consists of a lock stud member having a head on one end thereof for engaging said azimuth ring groove, upwardly extending protrusions carried by said lock stud head for engaging the edge of said azimuth ring, a clamp nut carried by the opposite end of said lock stud member, and a lever member engaging said clamp nut for selectively imparting adjustments thereto for applying pressure to lock said azimuth ring to said protractor housing.

3. A protractor for a drafting machine as set forth in claim 1 in which the pressure applying means includes a lock stud member, a head carried on one end of said lock stud member and upwardly extending protrusions for engaging the edge of said azimuth ring carried by the head and extending in the same direction as the lock stud member from the head.

4. In a drafting machine protractor assembly, a protractor housing, a stationary index ring having recesses secured to said protractor housing, an azimuth ring member, a hand knob assembly including a center post connecting the hand knob assembly to said azimuth ring member, said hand knob assembly and said azimuth ring member commonly journalled to said protractor' housing, a lifting plate disposed in guided engagement about said hand knob assembly center post, at least a pair of depending index pins carried by said lifting plate, and spring means included in said hand knob assembly to downwardly bias said lifting plate and maintain said index pins in the recesses of said index ring to lock said hand knob assembly from rotative movement with respect to said protractor housing.

5. In a drafting machine protractor assembly as set forth in claim 4, an incline plane member included in said hand knob assembly disposed about said center post, and a horizontally movable opposed coacting incline plane member connected intermediate said first incline plane member and said lifting plate for selectively raising said lifting plate and said index pins from locked engagement Wth the index ring recesses to permit rotation of said hand knob assembly and said azimuth ring With respect to said protractor housing.

6. In a drafting machine protractor assembly as set forth in claim 4, an incline plane member included in said hand knob assembly disposed about said center post, a trigger member, an opposed coacting incline plane portion carried by said trigger member, said trigger member connected intermediate said first incline plane member and said lifting plate and having a slot therein to receive saidcenter post, and said trigger member being horizon-tally shiftable for selectively raising said lifting plate and said index pins from locked engagement with the index ring recesses to permit rotation of said hand knob assembly and said azimuth ring with respect to said protractor housing.

'7. In a drafting machine protractor assembly as set forth in claim 4, an incline plane member included in said hand knob assembly disposed about said center post, a trigger member having a portion protruding from one side of said hand knob assembly, an opposed coacting incline plane portion carried by said trigger member and connected intermediate said first incline plane member and said lifting plate, said trigger member having a slot therein connected in guiding engagement with said center post, said trigger member being horizontally shiftable to raise said lifting plate and said index pins from locked engagement with the index ring recesses by movement of said incline plane portion with respect to said first incline plane member, said trigger member having a notch in one side thereof, and said trigger member being horizontally rotatable to engage a portion of said hand knob assembly in said notch to lock said index pins in the raised position to permit rotation of said hand knob assembly and said azimuth ring with respect to said protractor housing.

8. A drafting machine protractor assembly including, a housing member, an index ring having upwardly-facing radial recesses connected to said housing member, an azimuth ring disposed beneath said housing member, a hand knob assembly disposed above said index ring, means including a shaft member connecting said azimuth ring and said hand knob assembly journaled for rotative movement with respect to said housing member, selectively retractible index pins connected for vertical axial movement depending from said hand knob assembly into engagement with the upwardly-facing radial index ring recesses, and means connected to said index pins for lateral horizontal movement to raise and lower said index pins for selectively locking said hand knob assembly and azimuth ring to said housing member at selected azimuth settings.

References Cited in the file of this patent UNITED STATES PATENTS 1,297,788 Buck Mar. 18, 1919 2,627,323 Larson Feb. 3, 1953 2,900,728 Wallace Aug. 25, 1959 2,924,299 Kanzelberger Feb. 9, 1960 FOREIGN PATENTS 496,573 Great Britain Dec. 5, 1938 2,401,628 Switzerland May 11, 1946 321,083 Sweden June 19, 1951 752,154 Great Britain July 4, 1956 OTHER REFERENCES Germany (printed application) 13,793, Dec. 27, 1956. 

4. IN A DRAFTING MACHINE PROTRACTOR ASSEMBLY, A PROTRACTOR HOUSING, A STATIONARY INDEX RING HAVING RECESSES SECURED TO SAID PROTRACTOR HOUSING, AN AZIMUTH RING MEMBER, A HAND KNOB ASSEMBLY INCLUDING A CENTER POST CONNECTING THE HAND KNOB ASSEMBLY TO SAID AZIMUTH RING MEMBER, SAID HAND KNOB ASSEMBLY AND SAID AZIMUTH RING MEMBER COMMONLY JOURNALLED TO SAID PROTRACTOR HOUSING, A LIFTING PLATE DISPOSED IN GUIDED ENGAGEMENT ABOUT SAID HAND KNOB ASEMBLY CENTER POST, AT LEAST A PAIR OF DEPENDING INDEX PINS CARRIED BY SAID LIFTING PLATE, AND SPRING MEANS INCLUDED IN SAID HAND KNOB ASSEMBLY TO DOWNWARDLY BIAS SAID LIFTING PLATE AND MAINTAIN SAID INDEX PINS IN THE RECESSES OF SAID INDEX RING TO LOCK SAID HAND KNOB ASSEMBLY FROM ROTATIVE MOVEMENT WITH RESPECT TO SAID PROTRACTOR HOUSING. 